(Proposed Key Combinations):
The most likely method would be similar to W, Ctrl+W, Shift+W.
Land Vehicles/Boats:
Q, W, E /Ctrl+W, W, Shift+W = Forward
Ctrl+S, S, Shift+S = Reverse/Brakes
Airplanes:
Q (Why not Ctrl/Shift+Q as well?) = Accelerate
Ctrl+Z, Z, Shift+Z = Air brakes
Alternative Keys for land vehicles:
Z, X, C = Braking [These keys might not be currently used?]